A metal melting paste is known as one of conductive pastes to be used for filling a hole of a board and so on. The paste is obtained by mixing a conductive filler with a flux, a curing agent and a thermosetting resin, and the resin is cured and metal powders are fused and metallized by a means that the paste is heated under certain conditions (for example, Patent Document 1). In such a paste, metal powders themselves are integrated with each other as well as a metal powder is integrated with an end surface of a conductive layer in a via hole. Therefore, high conductivity is obtained in comparison with the case where metal powders are merely brought into contact with each other or the metal powder is merely brought into contact with an end surface of a conductive layer, and reliability of connection at the end surface of the conductive layer is also remarkably improved.
However, in the conventional metallized paste, a silver powder or a silver-coated copper powder has been used as the metal powder. Good conductivity is obtained by using these powders but a silver powder is expensive. Furthermore, there is a problem that long-term reliability is not sufficient because it is difficult to coat the copper powder entirely with silver.
On the other hand, the conductive paste is required to have high storage stability and long pot life.
In recent years, a silver-coated copper alloy powder obtained by coating the surface of a copper alloy with silver is put into practical use as a metal powder that is inexpensive and has improved storage stability in comparison with a silver powder (for example, Patent Document 2).
However, even though such a silver-coated copper alloy powder is used in the preparation of a conductive paste, it does neither prolong pot life of the paste nor improve conductivity of a cured product and its long-term reliability.